Siliceous algae-based seasonal temperature inference and indicator pollen tracking ca. 4,000 years of climate/land use dependency in the southern Austrian Alps

Diatom and chrysophyte cyst-based reconstructions of the dates of spring and autumn lake-mixing enabled us to estimate spring (ST_anom) and autumn (AT_anom) temperature anomalies as well as ice-cover of the last ca. 4,000 years in a lake sediment core (Oberer Landschitzsee, 2,076 m a.s.l.) from the southern slopes of the Austrian Central Alps. The two independently inferred temperature anomalies were significantly correlated. On average, spring and autumn temperatures were lower during the two millennia B.C than during 0–1,300 A.D. Marked spring and autumn temperature minima occurred at about 1,300 and 600 B.C. At about 1,300 A.D, ST_anom declined again. Spring-temperature anomalies during Roman and Medieval times equaled or slightly exceeded the modern values and paralleled tree-line and glacier fluctuations. The de-coupling of autumn and spring climates, which began during the Medieval period, might indicate changes in major circulation modes. It was assumed that the North-Atlantic influence, triggering winter-rain climate in the Northern Mediterranean, became weaker during Medieval times, resulting in a trend towards warmer autumns and overall more continental climate conditions in the study area. Four pulses of land use, inferred from indicator pollen, occurred during (1) the Early to Late Bronze, (2) the transition from Late Bronze to Early Iron Age (Hallstatt), (3) Late Iron Age (La Tène, Celtic time) to Roman times, and (4) during high to late Medieval times. Climate seemed to be an important, though complex, trigger of Alpine land use.     

Climatic Changes from 12,000 to 4,000 Years Ago in the Austrian Central Alps Tracked by Sedimentological and Biological Proxies of a Lake Sediment Core

Major and trace elements, minerals, and grain-size were analysed from the early to mid-Holocene (12 to 4 ky BP) period of a sediment core from the Alpine lake Oberer Landschitzsee (ObLAN, 2076 m a.s.l.), which is located on predominantly crystalline bedrock on the southern slopes of the Austrian Central Alps. Geochemistry and mineralogy were compared with diatom-inferred (Di-) ‘date of autumn mixing’ (A_mix), DOC, pH, and selected indicator pollen species from the same sediment core. Principal components analysis (PCA) indicated a positive correlation between processes triggered by temperature and precipitation (e.g., lake mixing, DOC). PCA grouped indicators of physical weathering and enhanced catchment run-off (sand, quartz, feldspar), elements of weathering (e.g., Ti, Rb, Mn) under dryer conditions (clay to silt fractions), and elements that probably were related to changes in redox conditions (Cu, Fe, S, Zn). The duration and height of the snow-pack played an important role in this high-alpine environment, affecting weathering, erosion, pH, and lake stratification. Low Alnus viridis pollen abundance, together with markers for increased elements of erosion, indicated extensive snow-pack. Changes in S coupled with As, and elements indicating increased weathering, reflected climate oscillations. LOI was affected by productivity and erosion. High (late) Di–A_mix coupled with increased Di-DOC indicated prolonged summers with increased productivity. Cold and wet (snow-rich) phases and subsequent melting caused low pH and a decoupling of the significant linear correlation between sedimentary Ca and Di-pH. Weathering and leaching during climate deteriorations opposed the long-term trend in a loss of cations and forced in-lake alkalinity generation during the following lake warming. Overall, the multi-proxy study indicated complex climate-driven processes within different time-scales (long-term trends, climate oscillations, seasonality). The climate oscillations within 12–5 ky BP corresponded well with the cool and wet phases known from central Europe suggesting a dominant common Atlantic climate impact. When Mediterranean climate established between 5 and 4 ky BP, its influence on the southern slopes of the Alps increased.     

Regional heterogeneity and the effects of land use and climate on 20 lakes in the big woods region of Minnesota

Few studies have assessed the relative importance of landscape, land use history, climate, and regional heterogeneity on lake ecosystem processes, despite the likelihood that interactions among these factors must be important for controlling lake dynamics. We used 14 sediment measures from 20 lake cores in a climatically sensitive region of the prairie-forest border in southern Minnesota to (1) assess relationships between modern lake productivity (Carlson’s Trophic State Index [TSI]), modern land-use, catchment, and lake morphometry, and (2) contrast regional responses to climatic transitions from the ‘Medieval Climatic Anomaly’(1000–1350) to the ‘Little Ice Age’ (1350–1800) to ‘Modern’(~1980–1996 AD). TSI was significantly positively correlated with modern sedimentation rate, and accumulation rates of organic matter (OM), biogenic silica (BSi), and total phosphorus (TP). TSI was not significantly correlated with “modern” land cover, catchment, or lake morphometry characteristics, but total organic N(N) was negatively correlated with percent cultivation in the catchment area and negatively correlated with δ¹⁵N of bulk organic matter in sediment. Regional, among-lake heterogeneity was high over the past 1,000 years, but Little Ice Age (LIA) cooling appeared to result in an approximately 20% decline in OM, BSi and TP accumulation, while warming and cultural eutrophication of the past 150 years corresponded to a 200–400% increase in accumulation rates as well as an 80% increase in carbonates and a small but significant 10% drop in C/N ratios consistent with greater in-lake productivity. Our results indicate that climate does have regional effects on lake ecosystems but that among-lake variability is high, reflecting the importance of local factors and suggesting a need for (1) more explicit definition of what ‘regional’ means, (2) a focus on degree as well as direction of change, and (3) estimating accumulation rates based on multiple lakes and multiple cores within lakes.     

Sediment geochemistry of Lake Daihai,north-central China: implications for catchment weathering and climate change during the Holocene

A 12.87-m-long sediment core was retrieved from closed-basin Lake Daihai in the monsoon–arid transition zone of north-central China. Oxides of major elements and their ratios normalized to Al in the AMS-¹⁴C-dated core were employed to evaluate chemical weathering intensity (CWI) in the lake drainage basin, which reflects hydrothermal conditions in the study area. Lower CWI periods occurred prior to 14.5 ka BP, and during the intervals ca. 11.7–10.3, 3.5–3.2, 2.6–1.7 ka BP, and 1.2–0 ka BP, indicating relatively low temperatures and moisture availability. Greater CWI during the intervening periods ca. 14.5–11.7, 10.3–9.0, 3.2–2.6, and 1.7–1.2 ka BP, with the maximum CWI at ca. 6.7–3.5 ka BP, imply ameliorated hydrothermal conditions in the lake basin, i.e. higher temperatures and precipitation. Exceptionally low CWI, associated with high CaO/MgO ratio during ca. 9.0–6.7 ka BP, suggests higher evaporation rates in the area under warmer temperature. Overall, CWI displays in-phase variations with changes in organic matter (TOC, TN), carbonate (CaCO₃) and pollen assemblages, all of which are related to variations in monsoon effective precipitation. High CWI indicates strong monsoon-induced precipitation, whereas low CWI reflects a weak precipitation regime. The optimum hydrothermal status, recorded by the strongest CWI and maximum monsoon effective precipitation during ca. 6.7–3.5 ka BP defines the Holocene climate optimum (HCO) in the Lake Daihai region. These results indicate that the HCO prevails after the early Holocene in the monsoon–arid transition zone of north-central China. Temperature and precipitation variations during most of the Holocene, inferred from the lake sediments, are due largely to insolation forcing. Dry but warm conditions ca. 9.0–6.7 ka BP, however, probably reflect the complex interactions between insolation and geography (e.g. altitude and local topography).     

Climate and human impact on a meromictic lake during the last 6,000 years (Montcortès Lake,Central Pyrenees,Spain)

Sedimentological, mineralogical and compositional analyses performed on short gravity cores and long Kullenberg cores from meromictic Montcortès Lake (Pre-Pyrenean Range, NE Spain) reveal large depositional changes during the last 6,000 cal years. The limnological characteristics of this karstic lake, including its meromictic nature, relatively high surface area/depth ratio (surface area ~0.1 km²; z _max = 30 m), and steep margins, facilitated deposition and preservation of finely laminated facies, punctuated by clastic layers corresponding to turbidite events. The robust age model is based on 17 AMS ¹⁴C dates. Slope instability caused large gravitational deposits during the middle Holocene, prior to 6 ka BP, and in the late Holocene, prior to 1,600 and 1,000 cal yr BP). Relatively shallower lake conditions prevailed during the middle Holocene (6,000–3,500 cal years BP). Afterwards, deeper environments dominated, with deposition of varves containing preserved calcite laminae. Increased carbonate production and lower clastic input occurred during the Iberian-Roman Period, the Little Ice Age, and the twentieth century. Although modulated by climate variability, changes in sediment delivery to the lake reflect modifications of agricultural practices and population pressure in the watershed. Two episodes of higher clastic input to the lake have been identified: 1) 690–1460 AD, coinciding with an increase in farming activity in the area and the Medieval Climate Anomaly, and 2) 1770–1950 AD, including the last phase of the Little Ice Age and the maximum human occupation in late nineteenth and early twentieth centuries.     

Records of environmental and climatic changes during the late Holocene from Svalbard: palaeolimnology of Kongressvatnet

A multi-core, multidisciplinary palaeolimnological study of the partially varved sediment of a deep, meromictic, arctic lake, Kongressvatnet (Svalbard, Western Spitsbergen), provides a record of environmental and climatic changes during last ca. 1800 years. The chronology of sedimentation was established using several dating techniques (¹³⁷Cs, ²¹⁰Pb, varve counts, palaeomagnetic correlation). A multiproxy record of palaeolimnological variability was compiled based on sedimentation rates, magnetic properties, varve thickness, organic matter, geochemistry, pigments from algal and photosynthetic bacteria, mineralogy and biological assemblages (diatoms, Cladocera). The major features recognised in our master core K99-3 include a shift in sediment source and supply (magnetic measurements, geochemistry) probably caused by glaciological changes in the catchment around 38–32 cm core depth (AD 700–820). Additional environmental changes are inferred at 20–18, 8–4.5 and 3–2 cm (AD ca. 1160–1255; 1715–1880; 1940–1963, respectively). During the past ca. 120 years a prominent sedimentological change from brownish-grey, partly laminated silt-clay (varves) to black organic-rich deposits was observed. From AD 1350 to AD1880 the sediment is comprised of a continuous sequence of varves, whereas the earlier sediments are mostly homogeneous with only a few short intercalated laminated sections between AD 860 and 1350. Sedimentation and accumulation rates increased during the last 30 years (modern warming). Pigment concentrations are very low in the lower ca. 32 cm of the core (AD 820) probably because of the high turbidity high energy environment. The high sulphur content in the uppermost 32 cm of sediment has given rise to two horizontally stratified populations of sulphur anaerobic photosynthetic bacteria, as inferred from their specific carotenoids. These bacteria populations are much more abundant during the Little Ice Age (LIA) than during warmer periods (e.g., during the Medieval Warm Period and 20th century). Diatoms are lacking from the core base up to 18 cm (ca. AD 1255); at this level, species indicative of mesotrophic water are present, whereas from 17 cm to the top of the core, oligotrophic taxa such as Staurosira construens/S. pinnata complex dominate, indicating extended ice coverage and more oligotrophic waters during the LIA. The concentration of Cladocera subfossil remains (dominated by Chydorus) are relatively high in the deepest sections (54–32 cm), whereas the upper 32 cm are characterized by a very low concentration of remains, possibly because of the strongly anoxic conditions, and in this upper sediment section rotifer resting eggs become prevalent. We interpret these changes as responses to climate forcing through its impact on glacial melt water, lake ice cover duration and mainly redox conditions in deep water. The observed changes suggest that at least some of our recorded changes may parallel the Greenland Ice core, although our study added more details about the inferred climatic changes. Further aspects are discussed, such as catchment processes, glacial activity, duration of the Medieval Warm Period, the Little Ice Age, local human activity, and limnology.     

Late Holocene palaeoenvironmental evolution of the Roman harbour of <Emphasis Type="Italic">Portus</Emphasis>, Italy

We present a Holocene record of climate and environmental change in central New York (USA) inferred using lithologic and stable isotope data from two sediment cores recovered in Cayuga Lake. The record was divided into three intervals: (1) early Holocene (~11.6–8.8 ka), (2) Hypsithermal (~8.8–4.4 ka), and (3) Neoglacial (~4.4 ka to present). The early Holocene began abruptly, with rising lake level and relatively deep water. Between ~10.8 and 9.2 ka, cool and dry conditions prevailed at a time of maximum solar insolation. This anomaly has been referred to as the “post-Younger Dryas climate interval” and lasted ~1,600 years, the approximate length of one “Bond cycle.” The Hypsithermal was the warmest, wettest and most biologically productive interval of the Holocene in central New York. The Hypsithermal was characterized by centennial to multi-centennial-scale variability. The 8.2 ka event is one such variation. The Neoglacial was an interval of generally cooler and dryer conditions, falling lake levels, and several prominent climate anomalies. At approximately 2.4 ka, δ¹³C of bulk organic matter increased abruptly by 5‰ as lake level declined, and the lake flora was dominated by Chara sp. during the coldest interval of the Neoglacial. Numerous sediment variables display increased variability ~2.0 ka, which continues today. Archaeological data from the literature suggest that Native American populations may have been large enough to impact land cover by about 2.4 ka and we hypothesize that the “Anthropocene” began at about that time in central New York. We also found paleolimnological evidence for the Medieval Warm Period (~1.4–0.5 ka), which was warmer and wetter than today, and for the Little Ice Age (~500–150 years ago), a period with temperatures colder than today.     

Biogeochemical records of paleoenvironmental changes in Nainital Lake,Kumaun Himalayas,India

Rapid urbanization and increased tourism around Nainital Lake in the Kumaun Himalayan region in north India has raised concerns about sediment and water pollution. Lead-210 dated sediment cores from the lake represent ~95 years of accumulation and yield a mean sedimentation rate of ~4.7 mm year⁻¹. Total organic carbon (TOC), percent N and S and their atomic C/N and C/S ratios, stable isotopes (δ¹³C, δ¹⁵N, and δ³⁴S), and specific biomarkers (n-alkanes and pigments) were measured in the core. Organic matter is primarily derived from in-lake algal production and TOC flux varies from 1.0 to 3.5 g m⁻² year⁻¹. Sediments are anoxic (Eh −328 to −187 mV) and have low (0.10–0.30 g m⁻² year⁻¹) N, but high (0.37–1.0 g m⁻² year⁻¹) S flux. Shifts in δ¹³C, δ¹⁵N, and δ³⁴S suggest in-lake microbial processes dominated by denitrification and sulfate reduction. The sediments are dominated by short-chain hydrocarbons with low Carbon Preference Index values. The pigments indicate a gradual shift to cyanobacterial domination of the phytoplankton community in recent years. Despite an increase in external input of nutrients, the trophic state of the lake has remained largely unchanged, and the perceived human-induced impacts are limited.     

Middle and late Holocene climate change and human impact inferred from diatoms,algae and aquatic macrophyte pollen in sediments from Lake Montcortès (NE Iberian Peninsula)

During the middle and late Holocene, the Iberian Peninsula underwent large climatic and hydrologic changes, but the temporal resolution and regional distribution of available palaeoenvironmental records is still insufficient for a comprehensive assessment of the regional variability. The high sedimentation rate in karstic, meromictic Montcortès Lake (Catalan pre-Pyrenees) allows for a detailed reconstruction of the regional palaeoecology over the last 5,340 years using diatom analysis, aquatic pollen, sedimentological data, and historic documentary records. Results show marked fluctuations in diatom species assemblage composition, mainly between dominant Cyclotella taxa and small Fragilariales. We suggest that the conspicuous alternation between Cyclotella comta and C. cyclopuncta reflects changes in trophic state, while the succession of centric and pennate species most likely reflects changes in the hydrology of the lake. The diatom assemblages were used to identify six main phases: (1) high productivity and likely lower lake levels before 2350 BC, (2) lower lake levels and a strong arid phase between 2350 and 1850 BC, (3) lake level increase between 1850 and 850 BC, (4) relatively high lake level with fluctuating conditions during the Iberian and Roman Epochs (650 BC–350 AD), (5) lower lake levels, unfavourable conditions for diatom preservation, eutrophication and erosion triggered by increased human activities in the watershed during the Medieval Climate Anomaly (900–1300 AD), and (6) relatively higher lake levels during the LIA (1380–1850 AD) and afterwards. The combined study of diatoms, algae and pollen provides a detailed reconstruction of past climate, which refines understanding of regional environmental variability and interactions between climate and socio-economic conditions in the Pyrenees.     

A ca. 800-Year Lithologic Record of Drought from Sub-annually Laminated Lake Sediment, East Java

Lithostratigraphic analyses of a sub-annually laminated core from Ranu Lamongan, a maar lake on the island of Java, document considerable changes in the lake’s chemistry and water balance over the past ca. 800 calendar years. Composition of the dark (clastics) and light (diatoms and/or calcium carbonate minerals) couplets suggests that these laminations form in response to seasonal changes in rainfall and water-column overturn in the lake. Calcium carbonate is not continuous in the core, and when it occurs it varies, sometimes abruptly, in carbonate phase and elemental composition (low Mg-calcite Mg-calcite, and aragonite). A significant correlation between Mg/Ca changes and δ¹⁸O variations in authigenic calcium carbonate suggest the basin is highly sensitive to hydrologic variation. Lithologic data suggest calcium carbonate precipitates and thus records hydrologic conditions during the dry season – a season in which rainfall anomalies are highly correlated with the phase of ENSO. Our carbonate-based record of Mg/Ca shows variability in evaporative concentration on a quasi-seasonal frequency for the past ca. 800 years. Our record shows two multi-decadal periods of drought – ca. 1275–1325 and ca. 1450–1650 CE – the latter of which was especially strong and/or prolonged. Our record also shows a possible change in drought frequency at around 1650 CE, in which periods of calcium carbonate precipitation and Mg/Ca change shifted from multi-decadal to interannual variability. Given the strong correlations between modern-day drought in East Java and ENSO variability, our drought record may indicate a regime shift in the behavior of the ENSO system about 350 years ago. Finally, comparisons between our record and others suggest that variation in ENSO on centennial and sub-centennial scales is not strongly associated with changes in the global mean climate state.     

Faunal (Chironomidae, Cladocera) responses to post-Little Ice Age climate warming in the high Austrian Alps

Present climate warming strongly affects limnological and ecological properties of lakes and may cause regime shifts that alter structure and function in the water bodies. Such effects are especially pronounced in climatologically extreme areas, e.g. at high altitudes. We examined a sediment core from Lake Oberer Landschitzsee, Austrian Alps, which spans the period from the Little Ice Age (LIA) to present. We investigated whether post-LIA climate warming altered aquatic invertebrate communities and limnological status in this sensitive high Alpine lake. Fossil Cladocera (Crustacea) and Chironomidae (Diptera) and organic matter in the core were analyzed. Chironomids were used to assess the lake??s benthic quality (i.e. oxygen availability). An instrumental Alpine temperature record was used to assess whether changes in the biotic assemblages correspond to post-LIA temperature trends. The planktonic and macro- and microbenthic invertebrate communities exhibit almost complete and simultaneous species turnover after the LIA, from about AD 1850 onward, when Sergentia coracina-type replaced oxyphilous Micropsectra contracta-type as the dominant macrobenthic taxon, and phytophilous Acroperus harpae outcompeted Alona affinis and Alona quadrangularis in the microbenthos. These directional community shifts corresponded with a period of reduced benthic quality, higher sediment organic content, and progressive climate warming, superimposed on Alpine land-use changes, until the early twentieth century. Detected changes suggest increased productivity and lower benthic oxygen availability. Faunal shifts were even more pronounced during the late twentieth century, simultaneous with enhanced warming. A new planktonic Cladocera species, Bosmina longirostris, typically absent from high Alpine lakes, colonized the lake and gradually became dominant toward the core top. Results show that post-LIA climate warming, coupled with increasing benthic and planktonic production, substantially altered the limnological and ecological status of this remote Alpine lake. Observed faunal turnovers provide evidence that temperature-driven ecological thresholds, whether associated directly or indirectly with greater human activity, have been crossed. Species abundances and distributions have changed in response to post-LIA and late twentieth century climate warming.     

An Early Pliocene lake and its surrounding vegetation in Zhejiang,East China

The palynomorph composition of an Early Pliocene assemblage from Du’ao Lake, Zhejiang Province, East China, including sporomorphs and algae, was analyzed to reconstruct the vegetation and climate around the lake, as well as the environmental conditions in the lake. A subtropical evergreen and deciduous broad-leaved mixed forest surrounding the lake is inferred from the pollen data. The composition of the green algae community indicates a clear, shallow (about 5–6 m deep), mesotrophic freshwater lake. The inferred pH was about 7.0–8.0 during the algae growing season. Applying the Coexistence Approach, the climatic conditions in Early Pliocene Du’ao were: (1) mean annual temperature ranged from 18.1 to 22.0°C, (2) difference in temperature between the coldest and warmest months ranged from 14.2 to 15.1°C, (3) mean temperature of the coldest month varied from 10.7 to 12.1°C, (4) mean temperature of the warmest month ranged from 23.5 to 25.4°C, (5) mean annual precipitation varied from about 994 to 1,255 mm, (6) minimum monthly precipitation ranged from about 9 to 11 mm, and (7) maximum monthly precipitation varied from approximately 219 to 245 mm. These values indicate that the Early Pliocene climate was subtropical.     

Multi-proxy evidence of postglacial climate and environmental change at Two Frog Lake,central mainland coast of British Columbia,Canada

Pollen and diatoms preserved in the radiocarbon dated sediments of Two Frog Lake in the Seymour-Belize Inlet Complex of the central mainland coast of British Columbia document postglacial climate change. Two Frog Lake was isolated from the sea prior to 11,040 ± 50 yr BP (13,030 cal. yr BP) when the climate was cool and dry, and open Pinus contorta woodlands covered the landscape. These woodlands were replaced by a mixed conifer forest ca. 10,200 yr BP (ca. 12,300 cal. yr BP) when the climate became moister. A relatively dry and warm early Holocene climate allowed Pseudotsuga menziesii to migrate northward to this site where it grew with Picea, Tsuga heterophylla and Alnus. The climate became cooler and moister at ca. 8,000 yr BP (ca. 9,200 cal. yr BP), approximately 500–1,000 years prior to sites located south of Two Frog Lake and on the Queen Charlotte Islands, but contemporary with sites on the northern mainland coast of British Columbia and south coastal Alaska. Climate heterogeneity in central coastal British Columbia appears to have occurred on a synoptic scale, suggesting that atmospheric dynamics linked to a variable Aleutian Low pressure system may have had an important influence on early Holocene climate change in the Seymour-Belize Inlet Complex. The transition to cooler and moister conditions facilitated the expansion of Cupressaceae and the establishment of a modern-type coastal temperate rainforest dominated by Cupressaceae and T. heterophylla. This was associated with progressive lake acidification. Diatom changes independent of vegetation change during the late Holocene are correlative with the mid-Neoglacial period, when cooler temperatures altered diatom communities.     

Holocene paleolimnological changes in Lake Skallen Oike in the Syowa Station area of Antarctica inferred from organic components in a sediment core (Sk4C-02)

Antarctic climate changes influence environmental changes at both regional and local scales. Here we report Holocene paleolimnological changes in lake sediment core Sk4C-02 (length 378.0 cm) from Lake Skallen Oike in the Soya Kaigan region of East Antarctica inferred from analyses of sedimentary facies, a range of organic components, isotope ratios of organic carbon and nitrogen, and carbon-14 dating by Tandetron accelerator mass spectrometry. The sediment core was composed of clayish mud (378.0–152.5 cm) overlain by organic sediments (152.5 cm-surface). The age of the surface and the core bottom were 150 (AD1950-1640) and ca. 7,030 ± 73 calibrated years before present (cal BP), respectively, and the mean sedimentation rate was estimated to be 0.55 mm/year. Multi-proxy analyses revealed that the principal environmental change in the core is a transition from marine to lacustrine environments which occurred at a depth of 152.5 cm (ca. 3,590 cal BP). This was caused by relative sea level change brought about by ongoing retreat of glaciers during the mid-Holocene warming of Antarctica, and ongoing isostatic uplift which outpaced changes in global (eustatic) sea level. The mean isostatic uplift rate was calculated to be 2.8 mm/year. The coastal marine period (378.0–152.5 cm, ca. 7,030–3,590 cal BP) was characterized by low biological production with the predominance of diatoms. During the transition period from marine to freshwater conditions (152.5-approximately 135 cm, ca. 3,590–3,290 cal BP) the lake was stratified with marine water overlain by freshwater, with a chemocline and an anoxic (sulfidic) layer in the bottom of the photic zone. Green sulfur bacteria and Cryptophyta were the major photosynthetic organisms. The Cryptophyta appeared to be tolerant of the moderate salinity and stratified water conditions. The lacustrine period (approximately 135 cm-surface, ca. 3,290 cal BP-present) was characterized by high biological production by green algae (e.g. Comarium clepsydra and Oedegonium spp.) with some contributions from cyanobacteria and diatoms. Biological production during this period was 8.7 times higher than during the coastal marine period.     

Climate-related cyclic deposition of carbonate and organic matter in Holocene lacustrine sediment,Lower Michigan,USA

Records from lake sediment cores are critical for assessing the relative stability of climate and ecosystems over the Holocene. Duck Lake in south-central Lower Michigan, USA, was the focus of a study that identified how changes in the geochemical variables in lake sediments relate to variations in regional climate and local land use during the Holocene. More than 8.5 m of lacustrine sediment were recovered using Livingston and freeze corers and analyzed for organic carbon, inorganic (carbonate) carbon, total nitrogen, and trace metals. Repeating packages of sediment (1–10 cm thick) that grade from light (inorganic carbon-rich) to dark (organic carbon-rich) were found from the surface to a depth of about 8 m. Variations in the high-resolution gray scale data from core X-radiographs are highly correlated to the relative amount of inorganic carbon. Geochemical analyses of the upper 8.5 m of sediment revealed a wide range of values: 0.05–10.6% for inorganic carbon (i.e. 0.5–89% calcium carbonate) and 1.1–28% for organic carbon (i.e. 2.7–70% organic matter). Organic carbon to nitrogen ratios indicate that most of the sediment organic matter is produced within the lake. A core chronology based on eight AMS radiocarbon dates shows low sediment accumulation rates (0.05 cm/year) from 10,000 to 3,800 cal year BP and higher sediment accumulation rates (0.1–0.3 cm/year) from 3,800 cal year BP to present. We suggest that carbonate accumulates during relatively dry times, whereas organic matter accumulation dominates when nutrient input to the lake is enhanced by wetter climate. The Duck Lake core records a distinct low point in inorganic carbon deposition that may be related to the 8.2 ka cooling event now documented from several sites in North America. Spectral analysis of gray scale values shows significant ~200-year periodicities over the past 8,000 years, hypothesized to result from climate changes induced by solar forcing. Concentrations of trace metals (e.g. lead, iron, copper, zinc) indicate the onset of regional anthropogenic influence about 150 cal year BP.     

Climate change and human impact at Sacrower See (NE Germany) during the past 13,000 years: a geochemical record

Lacustrine sediments in north-eastern Germany have rarely been used as archives to address the effects of climate change and human impact on both lake ecosystem and landscape evolution for this region. Sacrower See, a hardwater lake located in Brandenburg, provides a unique sediment record covering the past 13,000 years which was used to reconstruct climatic and anthropogenic forcing on lacustrine sedimentation. Time control is provided by 12 AMS ¹⁴C dates of terrestrial plant remains, the Laacher See Tephra, and the onset of varve formation in AD 1870 (80 cal. BP). Geochemical (including XRF logging of major elements, CNS analyses as well as δ¹³C_org and δ¹⁵N measurements) and pollen analyses allowed detecting detailed environmental changes in the sediment record. During the Younger Dryas cold phase increased soil erosion and hypolimnetic oxygen depletion enhanced the nutrient supply to the lake water causing eutrophic conditions. The beginning of the Holocene is characterized by large changes in C/N ratios, total sulphur, δ¹³C of bulk organic matter as well as in K, Si, and Ti, reflecting the response of the lake’s catchment to climatic warming. Reforestation reduced the influx of detrital particles and terrestrial organic matter. The first, rather weak evidence of human impact is documented only in the pollen record at 5,500 cal. BP. However, until 3,200 cal. BP sedimentological and geochemical parameters indicate relatively stable environmental conditions. During periods of intense human impact at around 3,200, 2,800, and 900 cal. BP peaks in Ti and K represent phases of increased soil erosion due to forest clearing during the Bronze Age, Iron Age, and Medieval Times, respectively. In general, greater variation is observed in most variables during these perturbations, indicating less stable environmental conditions. The steady rise of biogenic silica accumulation rates during the Holocene reflects an increasing productivity of Sacrower See until diatoms were outcompeted by other algae during the last centuries. The applied multi-proxy approach fosters the interpretation of the sediment record to reveal a consistent picture of environmental change including environmental factors controlling lake ontogeny and the effects of human impact.     

Late Pleistocene: Holocene record of environmental changes in Lake Zirahuen,Central Mexico

Geochemical data obtained from X-ray fluorescence, physical properties, total organic and inorganic carbon content (TOC/TIC), and diatom analysis from a 6.61-m-long sedimentary sequence near the modern northern shore of Lake Zirahuen (101° 44′ W, 19° 26′ N, 2000 m asl) provide a reconstruction of lacustrine sedimentation during the last approximately 17 cal kyr BP. A time scale is based on ten AMS ¹⁴C dates and by tephra layers from Jorullo (AD 1759-1764) and Paricutin (AD 1943-1952) volcanoes. The multiproxy analyses presented in this study reveal abrupt changes in environmental and climatic conditions. The results are compared to the paleo-record from nearby Lake Patzcuaro. Dry conditions and low lake level are inferred in the late Pleistocene until ca. 15 cal kyr BP, followed by a slight but sustained increase in lake level, as well as a higher productivity, peaking at ca. 12.1 cal kyr BP. This interpretation is consistent with several regional climatic reconstructions in central Mexico, but it is in opposition to record from Lake Patzcuaro. A sediment hiatus bracketed between 12.1 and 7.2 cal kyr BP suggests a drop in lake level in response to a dry early Holocene. A deeper, more eutrophic and turbid lake is recorded after 7.2 cal kyr BP. Lake level at the coring site during the mid Holocene is considered the highest for the past 17 cal kyr BP. The emplacement of the La Magueyera lava flows (LMLF), dated by thermoluminiscence at 6560 ± 950 year, may have reduced basin volume and contributed to the relative deepening of the lake after 7.2 cal kyr BP. The late Holocene (after 3.9 cal kyr BP) climate is characterized by high instability. Extensive erosion, lower lake levels, dry conditions and pulses of high sediment influx due to high rainfall are inferred for this time. Further decrease in lake level and increased erosion are recorded after ca. AD 1050, at the peak of Purepechas occupation (AD 1300–1521), and until the eighteenth century. Few lacustrine records extend back to the late Pleistocene—early Holocene in central Mexico; this paper contributes to the understanding of late Pleistocene-Holocene paleoclimates in this region.     

Hydrological change in the central interior of British Columbia,Canada: diatom and pollen evidence of millennial-to-centennial scale change over the Holocene

Diatom-based inferences of post-glacial hydrological change from a sedimentary record from Felker Lake, British Columbia, show millennial-scale pacing of climate over the past approximately 11670 calendar years with change at ca. 8140 cal. year BP, ca. 6840 cal. year BP, ca. 5700 cal. year BP, and ca. 2230 cal. year BP. Early postglacial diatom assemblages are dominated by fragilaroid taxa, suggesting that cool and moist climate conditions and relatively high lake levels prevailed at this time. Early Holocene warming near ca. 8140 cal. year BP promoted Cyclotella bodanica var. lemanica, a fall bloomer competitive in limnological conditions associated with warmer water and stratified conditions. Short-lived peaks of Stephanodiscus parvus/minutulus between ca. 6340 cal. year BP and ca. 5860 cal. year BP indicate periodic increases in nutrient availability and prolonged mixing likely associated with long cool and moist spring seasons. The diatom-inferred depth of Felker Lake increased during the mid-Holocene to reach a record high-stand at ca. 5860 cal. year BP. Large changes in hydrological variability and terrestrial vegetation at Felker Lake occurred after ca. 2230 cal. year BP when high-amplitude centennial-scale fluctuations in diatom-inferred lake depth and salinity are observed. Change is first documented in terrestrial vegetation at this time by a shift from open Pinus parklands to a landscape that periodically supported populations of Cupressaceae. Three record low-stand high-salinity events are reconstructed between ca. 1910 cal. year BP and ca. 1800 cal. year BP, ca. 1030 cal. year BP and ca. 690 cal. year BP, and ca. 250 cal. year BP and ca. 140 cal. year BP. The low lake-level episode of ca. 1030 cal. year BP–ca. 690 cal. year BP is coeval with the Medieval Warm Period (ca. 1000 cal. year BP–ca. 600 cal. year BP), a period of intense drought in western North America. Post-glacial hydrological change at Felker Lake is coherent with regional, hemispherical, and global paleoclimate events, suggesting that millennial-and centennial-scale shifts in water availability are a persistent feature of the climate of western North America.     

A model for inferring dissolved organic carbon (DOC) in lakewater from visible-near-infrared spectroscopy (VNIRS) measures in lake sediment

We developed an inference model to infer dissolved organic carbon (DOC) in lakewater from lake sediments using visible-near-infrared spectroscopy (VNIRS). The inference model used surface sediment samples collected from 160 Arctic Canada lakes, covering broad latitudinal (60–83°N), longitudinal (71–138°W) and environmental gradients, with a DOC range of 0.6–39.6 mg L⁻¹. The model was applied to Holocene lake sediment cores from Sweden and Canada and our inferences are compared to results from previous multiproxy paleolimnological investigations at these two sites. The inferred Swedish and Canadian DOC profiles are compared, respectively, to inferences from a Swedish-based VNIRS-total organic carbon (TOC) model and a Canadian-based diatom-inferred (Di-DOC) model from the same sediment records. The 5-component Partial Least Squares (PLS) model yields a cross-validated (CV) R_CV² R_{CV}^{2}  = 0.61 and a root mean squared error of prediction (RMSEP _CV ) = 4.4 mg L⁻¹ (11% of DOC gradient). The trends inferred for the two lakes were remarkably similar to the VNIRS-TOC and the Di-DOC inferred profiles and consistent with the other paleolimnological proxies, although absolute values differed. Differences in the calibration set gradients and lack of analogous VNIRS signatures in the modern datasets may explain this discrepancy. Our results corroborate previous geographically independent studies on the potential of using VNIRS to reconstruct past trends in lakewater DOC concentrations rapidly.     

Sedimentary geochemistry of core PG1351 from Lake El’gygytgyn—a sensitive record of climate variability in the East Siberian Arctic during the past three glacial–interglacial cycles

The ca. 13 m long sediment core PG1351, recovered in 1998 from the central part of Lake El’gygytgyn, NE Siberia, was investigated for lithostratigraphy, water content, dry bulk density (DBD), total organic carbon (TOC), total nitrogen (TN), total sulphur (TS) and biogenic silica (opal) contents, and for TOC stable isotope ratios (δ¹³C_TOC). The event stratigraphy recorded in major differences in sediment composition match variations in regional summer insolation, thus confirming a new age model for this core, which suggests that it spans the last 250 ka BP. Four depositional units of contrasting lithological and biogeochemical composition have been distinguished, reflecting past environmental conditions associated with relatively warm, peak warm, cold and dry, and cold but more moist climate modes. A relatively warm climate, resulting in complete summer melt of the lake ice cover and seasonal mixing of the water column, prevailed during the Holocene and Marine Isotope Stages (MIS) 3, 5.1, 5.3, 6.1, 6.3, 6.5, 7.1–7.3, 7.5, 8.1 and 8.3. MIS 5.5 (Eemian) was characterized by significantly enhanced aquatic primary production and organic matter supply from the catchment, indicating peak warm conditions. During MIS 2, 5.2, 5.4, 6.2 and 6.4 the climate was cold and dry, leading to perennial lake ice cover, little regional snowfall, and a stagnant water body. A cold but more moist climate during MIS 4, 6.6, 7.4, 8.2 and 8.4 is thought to have produced more snow cover on␣the perennial ice, strongly reducing light penetration and biogenic primary production in␣the lake. While the cold–warm pattern during␣the past three glacial–interglacial cycles is probably controlled by changes in regional summer insolation, differences in the intensity of the warm phases and in the degree of aridity (changing snowfall) during cold phases likely were due to changes in atmospheric circulation patterns. This is the seventh in a series of eleven papers published in this special issue dedicated to initial studies of El'gygytgyn Crater Lake and its catchment in NE Russia. JulieBrigham-Grette, Martin Melles, Pavel Minyuk were guest editors of this special issue.